This invention relates to a discharge lamp apparatus for starting and operating the discharge lamp at a high frequency.
This invention also relates to allowed U.S. application Ser. No. 288,133 filed Dec. 22, 1988 entitled "DISCHARGE LAMP OPERATION APPARATUS" (inventor: Masakatsu KICHIBAYASHI et.al.)
Recently, in discharge lamp apparatus, attempts have been made to operate the lamp by means of a high frequency inverter which is a switching circuit composed of semiconductor devices, from the aspects of higher efficiency and smaller size and lighter weight. More lately, in particular, a compact fluorescent lamp has been developed, and a smaller size is demanded in the discharge lamp operation apparatus using a high frequency inverter.
The discharge lamp apparatus prior to this invention was composed, for example, of a circuit as shown in FIG. 1. This circuit was disclosed in Japanese Patent application No. 62-328596.
That is, in FIG. 1, numeral 4 denotes a power source circuit comprising a commercial power source 1, a rectifying bridge 2, and a smoothing capacitor 3, which outputs a D.C. voltage. Numeral 5 denotes a capacitor connected in series to the output of the power source circuit 4, and numeral 6 denotes a transistor connected between the capacitor 5 and the power source circuit 4; a series circuit of fluorescent lamp 7 and inductance 8 is connected in parallel to the capacitor 5, and a series circuit comprising a starting circuit composed of a capacitor 9 and an inductor 10 and a PTC thermistor 21 is connected in parallel the terminal of the fluorescent lamp not connected to the power source circuit 4. The PTC thermistor 21 is connected to the terminal of the lamp not connected to the transistor 6. Numeral 22 denotes a zener diode which is a voltage responsive switching element having one terminal connected to a connecting point A between the PTC thermistor 21 and the capacitor 9, and numeral 23 denotes a short-circuit transistor having a base connected to the diode 21A connected to the other terminal of the zener diode 22, and having a collector connected to a capacitor 13. Numeral 11 denotes a self-oscillation switching circuit composed of the capacitor 5, the transistor 6 and the inductor 8. Numeral 12 denotes a driving inductor having one terminal connected to the base of the transistor 6, and having a series circuit consisting of a secondary winding 8b of the inductor 8 and a driving capacitor 13 connected between the other terminal and an emitter of the transistor 6. Numeral 17 denotes a timer circuit composed of voltage dividing resistances 14 and 15 connected to an output terminal of the power source circuit 4 and a capacitor 16 having one terminal connected in the connecting point thereof; numeral 18 denotes a short-circuit transistor having a base connected to the other terminal of the capacitor 16 and having a collector and emitter connected in parallel to the inductor 12; numeral 19 denotes a diode having a cathode connected to the base of the transistor 6 and an anode connected to the emitter of the transistor 6 through a resistance 20.
The operation of the thus composed conventional circuit is explained below. When the power is turned on, a voltage is generated in the power source circuit 4, and a starting current flows through the resistor 14 of the timer circuit 17, capacitor 16 and the base of transistor 18 to cause transistor 18 to conduct, and at the same time the transistor 6 is turned on through its base current. Initially, the fluorescent lamp 7 is not operating, and the current flows from the power source circuit 4 and runs through the transistor 6 by way of the inductor 8, the filament electrode of fluorescent lamp 7, capacitor 9 and inductor 10. At this time, a positive voltage is generated in the secondary winding 8b of the inductor 8, and the base current of the transistor 6 is supplied through the capacitor 13 and inductor 12, so that the ON state of the transistor 6 is maintained. Here, the current flowing in the primary winding 8a of the inductor 8 is a resonance current of the capacitor 9 and inductor 8. At this moment, the current flowing due to the positive voltage generated in the secondary winding 8b of the inductor 8 is a series resonance current at the characteristic frequency of inductor 12 and capacitor 13, but actually since the transistor 18 is turned on, the current also flows slightly in the reverse direction from the emitter to the collector of the transistor 18, and the state of resonance is weak, and the inductor 12 hardly operates, and therefore, the oscillation period becomes shorter, being closer to the charge and discharge time of the capacitor 13. Accordingly, the capacitor 13 is charged nearly up to the voltage generated by the secondary winding 8b, and the base current of the transistor 6 no longer flows, and when the base current of the transistor 6 is slightly pulled in the reverse direction by the effect of the inductor 12, the transistor 6 is about to be turned off, and the voltage generated at the secondary winding 8b becomes small, so that the secondary winding 8b becomes small, so that the electric charge stored in the capacitor 13 is fed back to be applied in the reverse direction between the base and emitter of the transistor 6, thereby causing the transistor 6 to turn off quickly. When the transistor 6 turns off, the energy stored in the series resonance circuit of capacitor 9 and inductor 8, and the inductor 10 is released to the capacitor 5, fluorescent lamp 7, capacitor 9, inductor 8 and inductor 10, and oscillates to become a preheating current of the fluorescent lamp 7. At this time, the elements are arranged so that the fluorescent lamp 7 may not be started by the voltage generated by the capacitor 9. While the transistor 6 is turned off, the oscillation current flowing in the primary winding 8a of the inductor 8 generates a negative voltage at the secondary winding 8b of the inductor 8. Here, the transistor 18 is turned on in the forward direction, and the inductor 12 does not operate at all, and therefore, by this voltage, a reverse voltage is applied between the base and emitter of the transistor 6 through the diode 19 and resistor 20 to maintain the OFF state of the transistor 6. When the oscillation current passes over the negative peak, a positive voltage is gradually generated in the secondary winding 8b of the inductor 8, and the voltage charged in the reverse direction in the capacitor 13 while the transistor 6 is OFF is applied to the base of the transistor 6 in the forward direction to turn on the transistor 6. At this time, right after turning on, the current of the inductor 8 is still flowing in the reverse direction, and the current flows from the base to the collector through the diode 19 and resistance 20. Gradually the reverse current of the inductor 8 decreases, and a forward current begins to flow into the transistor 6, and thereafter the same operation as mentioned above is repeated. By this oscillation operation, as the time passes, the temperature of the preheat electrode of the fluorescent lamp 7 goes up.
The timer circuit 17 feeds base current to the transistor 18 while charging the capacitor 16 through the resistance 14 after the power source is turned on, and when charged up to the voltage specified by resistances 14 and 15 after a specified time, it is no longer charged, and thereafter the current is shut off, and the current no longer flows in the base of the transistor 18 until the electric charge in the capacitor 16 is discharged through the resistances 14 and 15. Accordingly, when the transistor 18 is turned off after a specified time, a positive voltage is generated by the secondary winding 8b of the inductor 8 when the transistor 6 is turned on, and a base current is supplied to the transistor 6 by way of the capacitor 13 and inductor 12. This base current is a resonance current of the capacitor 13 and inductor 12, and near the half period, the base current of the transistor 6 changes from positive to negative, and when the stored charge in the transistor 6 is released, the transistor 6 is turned off. Before starting the fluorescent lamp 7, the inductor 8 and capacitor 9 are in a series resonance state, and the inductors and capacitors are set so that a voltage which is much larger than the operating voltage, and larger than the preheating voltage before operation of timer circuit, and sufficient for starting the fluorescent lamp 7 is generated in the capacitor 9. Accordingly, the fluorescent lamp 7 is started. After starting, the operation of the circuit is almost similar to the operation after the start of the timer circuit 17, but since the impedance of the fluorescent lamp 7 is connected in parallel to the impedance of the capacitor 9 and inductor 10, the current of the capacitor 9 decreases, and a current flows in the fluorescent lamp 7. As a result, the resonance of the inductor 8 and capacitor 9 is almost lost, and a positive or negative voltage according to the difference of the output voltage of the power source circuit 4 and the lamp voltage is generated in the secondary winding 8b of the inductor 8, and the transistor 6 is controlled in its on/off operation depending on the characteristic frequency determined by the inductor 8, capacitor 5, fluorescent lamp 7, inductor 12, and capacitor 13. The inductor 10 is for the removal of DC components of the current of the fluorescent lamp 7. A transistor 23 is intended to stop the current for driving the transistor 6 from the driving circuit of the switching circuit 11 by the current flowing into the base thereof.
In the thus composed conventional circuit, when the electrode 7a of the fluorescent lamp 7 is emitterless and the electrode 7b has an emitter, by turning on the power source 1, the switching circuit 11 begins to oscillate. However, since the electrode 7a of the fluorescent lamp 7 is emitterless, the lamp current flows from the electrode 7a to the electrode 7b, but does not flow in the reverse direction. Accordingly, a large current corresponding to the lamp current flows in the PTC thermistor 21 through the capacitor 9, and the temperature of the PTC thermistor 21 goes up, and when nearing the curie temperature, the resistance increases suddenly. Along with the increase in the resistance value, the resonance voltage of the capacitor 9 and inductor 8 is overlaid on the supply voltage, and the peak voltage becomes high. When the peak voltage of the connecting point A reaches the operating voltage of the zener diode 22, a current flows into the base of the transistor 23 through the zener diode 22 from the connecting point A, and the transistor 23 is operated, and the connecting point B of the capacitor 13 and inductor 12 and the negative voltage side of the power source 4 are short-circuited. In consequence, the supply of base current to the transistor 6 is stopped, and the operation of the switching circuit stops.
On the other hand, when the electrode 7b of the fluorescent lamp 7 is emitterless and the electrode 7a has an emitter, and when both electrodes 7a and 7b are emitterless, the operation is as flows. In such cases, current hardly flows from the power source circuit 4 through the lamp directly, and the current directly passes through the PTC thermistor 21 from the power source circuit 4 to change the resistance of the PTC thermistor suddenly, and the operation of the switching circuit 11 is stopped by the transistor 23, while almost no current flows from the power source circuit 4 through the fluorescent lamp 7 and capacitor 9, and the base current of the transistor 6 does not flow due to the inductor 8, thereby stopping the operation of the switching circuit 11.
In such a constitution, however, when the lamp voltage becomes high after, for example, fluctuations of the supply voltage or dimming, the current flowing in the PTC thermistor 21 becomes large, and operation of the lamp may not be maintained although the PTC thermistor 21 is raised in temperature and the lamp is operating normally. Besides, when turning on the power, a large current always flows in the PTC thermistor 21, and the temperature of the PTC thermistor 21 is raised somewhat. Accordingly, when the power on/off operation is repeated, the lamp may not operate due to temperature rise of the PTC thermistor 21.